When a formation is drilled under normal conditions, the well is almost always filled with drilling fluid that serves to carry rock cuttings to the surface, lubricate the drill bit, and provide an overpressure in the borehole to prevent the flow of formation fluids into the wellbore (i.e., a blow out). The overpressure provided by the drilling fluid also plays a key role in stabilizing the formation. As a result of the overpressure, the liquid part of the drilling fluid enters the formation (filtering) while the solid part accumulates at the formation surface (borehole wall). The accumulated solid contains materials (such as bentonite, for example) that act to form a hydraulic seal. The sealing layer is called “mudcake” and, once formed, prevents any further filtering of the drilling fluid into the formation. Thus, although a relatively small volume of the drilling fluid filters into the formation, the process is normally self-limiting.
Mudcake is able to form and sealing occurs because the pore size in the subsurface formation is smaller than the particle sizes in the drilling fluid. As a result, the bulk of those particles cannot pass through the pore entrance (though a small portion of very fine particles can pass and produce what is known as “fine invasion”). The bulk of the solid drilling fluid material is pressed against and sticks to the pore entrance and gradually builds the impermeable layer of mudcake. However, this process fails to occur when the size of the pore entrance is larger than the solid particles in the mud (drilling fluid). One common example is when fractures are encountered. Some natural fractures have apertures larger than the particles in the mud. This results in a fluid loss problem wherein a large volume of drilling fluid is lost into the formation, with its consequential economic and safety issues.
Fluid loss in fractures is manageable and remedial actions exist. One such remedy is to use solid materials in the drilling fluid that are proportionally larger. With this approach pore sizes of up to 2.5 millimeters have been sealed. More recently, the use of water swellable materials has been proposed. In this case, smaller, water swellable materials are used in the formulation of the mud. These materials enter the fracture, absorb water, and increase their volume, thereby forming a seal. Certain water swellable materials are capable of increasing their weight by over ten-fold in the course of a few hours. The rate and extent of swelling depends on the type of water available. The best results are obtained with fresh water.
A “super k layer”, also known as a “cavernous formation”, is a source of huge permeability and, when encountered during drilling, can take in large volumes of drilling fluid, even to the point there is not enough drilling fluid left in the borehole to reach the surface. This is referred to as “circulation loss”. Because super k layers have very large pores (on the order of tens of centimeter), there is no possibility of forming a mudcake at the borehole wall. As a result, the fluid loss can continue indefinitely so long as the fluid pressure in the borehole is higher than the fluid pressure in the formation.